The length of scaffold which takes the sum length (summing from longest to shortest scaffold) past 50% of the total assembly size.

How many scaffolds are counted in reaching the N50 threshold.

The length of contig which takes the sum length (summing from longest to shortest contig) past 50% of the total assembly size.

How many contig are counted in reaching the N50 threshold.

A contig is a contiguous consensus sequence that is
derived from a collection of overlapping reads.
A scaffold is set of a ordered and orientated contigs
that are linked to one another by mate pairs of sequencing reads.